The effects of different relative humidities on some physical properties of three types of calfskin parchment (vellum) were investigated. Standard samples were subjected to (1) tensile fracture, and (2) measurement of the force that developed when the restrained samples were subjected to step decreases in relative humidity in the region between 60per thousand and 11per thousand. The results indicate that although no particular level of relative humidity can be excluded in general from consideration as a storage or display condition in the basis of tensile testing alone, at 11per thousand RH there is an adverse effect on some individual tensile properties. Relative humidities above 40per thousand increase gelation and opportunities for biological growth. About 25per thousand RH is the lowest level that can be tolerated without inducing large stresses in the material. On full consideration of these results, the physical chemistry and chemical reactivity of collagen, and the results of a recent study of the biodeterioration of parchment, a relative humidity of 30per thousand seems optimum for such papers. At 30per thousand RH, a cyclic variation of ±5per thousand can be permitted with minimal effects of swelling and shrinkage. -- CAL